However, cross-immunity within and between subtypes of influenza is poorly understood.

Often, the degree of cross-immunity between virus strains is assumed to be related to their sequence distance.

When the cross-immunity wears off the population is more susceptible to transmission whenever the next seasonal peak occurs.

In addition to risk of infection, cross-immunity may modulate the probability that a host becomes infectious and the duration that a host remains infectious.

One of the first compartmental models for influenza was developed by Gog and Grenfell, who simulated the dynamics of many strains with partial cross-immunity to one another.

Dr M. C. Agrawal demonstrated cross-immunity against Schistosoma incognitum by immunising the host against Schistosoma indicum.

Thus in yaws-endemic areas, where the disease is largely spread non-venereally in childhood, there is little opportunity for syphilis to spread, because of the degree of cross-immunity in the community.

Edward Jenner had also discovered vaccination, using cowpox to give cross-immunity to smallpox (in 1796), and by Pasteur's time this had generally replaced the use of actual smallpox material in inoculation.

For more complicated epidemiological models, such as those involving cross-immunity, age structure of host contact rates, seasonality, or multiple host populations with different life history traits, it is often impossible to analytically predict genealogical patterns from epidemiological parameters.

They showed that under strain-specific immunity alone (with partial cross-immunity between strains based on their amino acid similarity), the phylogeny of influenza A/H3N2's HA was expected to exhibit 'explosive genetic diversity', a pattern that is inconsistent with empirical data.